This invention relates to a process for stabilizing an organic peracid such as a percarboxylic acid, more specifically to a process for stabilizing a solution of a percarboxylic acid such as perpropionic acid, which is used for the reaction with cyclohexanone in the preparation of .epsilon.-caprolactone.
Among organic peracids which are typical peroxides, particularly a percarboxylic acid is an industrially important material, and is used for various chemical reactions such as epoxidation, hydroxylation, lactonization, formation of quinone, aromatic ring opening, formation of phenol and oxidation of ketone. However, the percarboxylic acid is an inherently unstable material. The percarboxylic acid is decomposed with violence while releasing oxygen due to elevated temperatures or contamination with impurities, whereby danger is incurred in some cases. Further, when left to stand, the percarboxylic acid is liable to be decomposed gradually while releasing oxygen even at room temperature. Thus, the instability is a serious drawback in using the percarboxylic acid.
When the percarboxylic acid exists in a pure state or coexists only with an inert substance, there is no stabilizer which enhances stability in that state. However, in a solution of the percarboxylic acid, particularly in an aqueous solution thereof, a trace amount of a metal ion is generally included, and the decomposition of the percarboxylic acid is accelerated by function of the metal ion as a decomposing catalyst. Therefore, as a stabilizer of the percarboxylic acid which can decrease the function of the metal ion as a decomposing catalyst existing in a solution of the percarboxylic acid, various stabilizers of the percarboxylic acid such as sequestering agents have been investigated.
More specifically, as the stabilizer as described above, there have been proposed stabilizers such as a polyphosphate (e.g. sodium pyrophosphate or sodium tripolyphosphate), ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, potassium thiocyanate, polyaminocarboxylic acid, a picolinic acid, a quinoline derivative, a dipicolinic acid (e.g. pyridine-2,6-dicarboxylic acid), pyridine-2,6-dimethanol, tributyl phosphate and alkyl pyrophosphate and they have been used singly or in combination thereof (see, for example, "Chemical Industry", Vol. 21, August, 1970 and "Special Editing, Organic Peracids and Peroxides--Utilization, Properties and Handling of Peracids" written by Katsutaro Yoshimatsu). Particularly, among these stabilizers, a phosphate such as tributyl phosphate, and a dipicolinic acid are effective, and it has been known that a dipicolinic acid is effective for a solution of the percarboxylic acid with a concentration of 10% by weight or more, but it can not necessarily be said to be satisfactory in the points of its effect and economy.